1. Field of the Invention
This invention relates to a film for cutting off the passage of heat rays through a transparent or semi-transparent material, such as glass or plastics, for e.g. the windows of vehicles, buildings, offices or houses, telephone booths, show windows, or lamps, and a coating liquid used for forming the same.
2. Description of the Prior Art
A sheet of glass having a film formed on its surface from a material reflecting visible and infrared wavelengths has been used as heat-ray reflecting glass for removing or reducing heat emitted by the sun, or an electric bulb, or any other source of light. The film has been formed from a material selected from among metal oxides such as FeOx, CoOx, CrOx and TiOx, and metals such as Ag, Au, Cu, Ni and Al.
These materials, however, not only reflect or absorb near-infrared radiation known as a source of heat, but also have a low transmittance of visible light. A transparent material used for buildings, vehicles, telephone booths, etc. are required to have a high transmittance of visible light. If any of the known materials as mentioned above is used for any such purpose, it has been necessary to form a very thin film, and it has been usual practice to employ a process, such as spraying and baking, CVD, sputtering or vacuum vapor deposition, to form a film having a very small thickness in the order of 10 nm.
These processes have, however, been unsatisfactory for a number of reasons including the necessity for a large apparatus, or vacuum equipment, low productivity, difficulty in forming a film covering a large area, and the expenditure of a large amount of cost.
Referring to the drawbacks of the films formed from the known materials, a film having a small thickness and thereby a high transmittance of light has a low property of cutting off heat rays, while a film is darkened if its thickness is sufficiently large to give it a high property of cutting off heat rays. A film having a high property of cutting off heat rays is also likely to have a high reflectivity of visible light and present a surface shining like a mirror and spoiled in appearance. Moreover, many of these materials give a film having a high electric conductivity which reflects radio waves and makes them unreceivable by a radio or television receiver, or a portable telephone, or causes the disturbance of waves in or around the structure in which the film is used.
These drawbacks can be overcome by a film having a low reflectivity of visible light and a high absorption or reflectivity of near infrared radiation, as well as a surface resistivity of or above about 10.sup.6 .OMEGA./.quadrature.. There has, however, not been known any such film, or any material suitable therefor.
Antimony tin oxide (ATO) and indium tin oxide (ITO) are known as materials having a high transmittance of visible light and a power of cutting off heat rays. These materials have a relatively low reflectivity of visible light and does not give a shining surface to a film, but as their plasma wavelengths fall in relatively long wavelengths in the near infrared region, a film formed from either material has been unsatisfactory in its property of reflecting or absorbing near infrared radiation close to visible light. Moreover, the film has been so high in electric conductivity as to reflect radio waves.